Wireless communication systems are widely used to provide voice and data services for multiple users using a variety of access terminals such as cellular telephones, laptop computers and various multimedia devices. Such communications systems can encompass local area networks, such as IEEE 801.11 networks, cellular telephone and/or mobile broadband networks. The communication system can use a one or more multiple access techniques, such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA) and others. Mobile broadband networks can conform to a number of system types or partnerships such as, General Packet Radio Service (GPRS), 3rd-Generation standards (3G), Worldwide Interoperability for Microwave Access (WiMAX), Universal Mobile Telecommunications System (UMTS), the 3rd Generation Partnership Project (3GPP), Evolution-Data Optimized (EV-DO), or Long Term Evolution (LTE).
Many wireless broadband network systems comprise base stations that serve mobile user devices. Within the wireless base station, the power amplifier (PA) is a key component. As more and more networks are deployed, there is an ever-increasing pressure to improve the efficiency of these amplifiers. Power efficiency is a measure of RF (AC) output power related to DC input power. As power amplifiers become more power efficient, the need for cooling requirements such as fans is reduced. Furthermore, reduced power amplifier power reduces the cost to operate the base station.
One way to achieve efficiency improvements in power amplifiers is by allowing the constituent power transistors to operate closer to saturation. Structures such as Doherty amplifiers allow for a given power transistor to operate near or at saturation over an extended output power range. This helps facilitate the efficient amplification of amplitude modulated signals with high peak-to-average-power ratios, as are found in modern wireless communication systems. The power efficiency improvement of traditional Doherty amplifiers over single ended class AB amplifiers is limited to bandwidths that are less than 20% of the carrier frequency. This often limits a Doherty amplifier to be used within a single designated frequency band. To transmit signals in multiple frequency bands simultaneously, multiple, separate and frequency specific amplifier modules that contain the Doherty structure in the final output stage are usually deployed.